An ultrasonic probe is used for a fish finder, an ultrasonic diagnostic equipment used with living bodies, or the like. As the ultrasonic equipment used for an ultrasonic diagnostic equipment, an ultrasonic probe including an ultrasonic element for transmitting and receiving ultrasonic waves, a sound window enclosing the ultrasonic element and a sound propagation liquid having a sound impedance, which is approximate to that of a living body, charged in the sound window is well known (see, for example, JP02 (1990)-98341 A).
Such ultrasonic probes use a resin for a material that constitutes the sound window, from the viewpoint of the sound properties. Therefore, since the water absorption rate is different depending upon the used resins due to the influence of the molecular structure, etc., after a long time of use, depending upon the kinds, temperature and resin materials, etc., of the sound propagation liquid, the sound propagation liquid may intrude into a resin that constitutes the sound window, or further may penetrate the sound window and leak toward the outside. As a result, the pressure inside the sound window is lowered so as to become lower than the external pressure, so that an air may penetrate the resin that constitutes the sound window and the air may be a contaminant inside the sound window. When air bubble contaminates into the sound window, it acts as a reflector with respect to the ultrasonic waves, thus inhibiting the transmittance and receiving of the ultrasonic waves, which in turn may lead to the deterioration of ultrasonic diagnostic images. In order to suppress an occurrence of such air bubbles, in this kind of ultrasonic probes, a sound propagation liquid has to be refilled over time so as to maintain the pressure inside the sound window.